Yes, this works too (it is actually the way I did it ) But it is best to lift your car and then just tighten the bar till it is "snug". The problem with preloading the bar while your car is on the ground is that you don't know when to stop tightening the bar. Basically, you want the strut bar to keep your frame neutral. i.e. the bar not pushing out on the frame and not pulling it in either.

You can also adjust your strut bar to give your car certain handling charactoristics for different racing conditions. But that is all over my head.